The invention relates to modeling photographic printing, and more particularly to modeling the process and apparatus for making a photographic print based on a photographic color negative.
Photographic printing produces a photographic print from a photographic negative. In the photographic printing process various factors combine to produce the resulting photographic print, including characteristics of the exposure illuminant and process, the print paper, and dyes. To accurately characterize a photographic print with a photographic printing model, each of these real aspects of the printing process needs to be modeled.
A scanner can be used to scan a photographic color negative and generate corresponding digital output values. Scanners produce digital output values to represent the spectral transmittance of an input sample. A typical scanner illuminates a transmissive target using a light source. The scanner integrates light that passes through the sample and passes through a set of spectrally selective filters. The integrated products may be modified by electronics and software to obtain digital output values. These digital output values can be combined with print and dye models to characterize photographic media. However, conventional photographic characterizations are based on density and are not channel independent. A channel independent characterization would provide a more flexible model.
In addition, different reflectance spectra can be perceived by the eye as the same color. Similarly, different spectra can produce the same digital values. This effect is called xe2x80x9cmetamerism.xe2x80x9d Metameric samples can produce different colorimetric response values when viewed or scanned under different viewing conditions. As a result, in modeling scanners, many different reflectance spectra can produce the same RGB values. To determine a more accurate estimate of a reflectance spectrum, it is desirable to limit the candidates to avoid metameric matches.
The invention provides methods and apparatus implementing a technique for predicting a reflectance spectrum of a photographic print. The technique models the print based on data from a source image, such as a color negative or color positive. The technique is also applicable to modeling an output image on transmissive media. The data is preferably obtained from an image acquisition device such as a scanner or digital camera. The technique uses dye concentration, providing a channel-independent basis for the spectral model. When used in combination with a color management system, the technique allows for previewing or manipulation of photographic images or experimentation with photographic processes via simulation. The user can control the simulated processing with an exposure schedule and optional print illuminant color temperature that controls the exposure time and spectral content of the printing illuminant. The user can also control other aspects of the simulated processing by modifying parameters such as the color filters, the illuminant spectrum, and the spectral sensitivities or absorption spectra of the paper.
The preferred spectral model includes three conceptual sections: scanner and film models, a photographic printer model, and a photographic paper model. The scanner and film section converts digital data from a scan of a color negative on a calibrated scanner into predictions of the spectral transmittance for each pixel. In alternative implementations, the scanner model is replaced with alternative models of image acquisition devices. The photographic printer section uses an exposure schedule and optionally a color temperature and computes the spectrum of the exposing illumination as a function of time. The photographic paper section converts the illuminated negative""s spectral power first into dye concentration estimates and subsequently into a predicted reflectance spectrum for the simulated print.
In general, in one aspect, the technique includes: converting digital values from scanning a photographic negative to a film transmittance spectrum using a photographic negative film model corresponding to a media of the photographic negative; estimating an exposure illumination spectrum over time using an exposure schedule; estimating paper spectral sensitivities of a photographic paper corresponding to the photographic print; integrating over time spectral products of the film transmittance spectrum, the exposure illumination spectrum, and the paper spectral sensitivities; converting the integrated spectral products into log integrated exposures; converting the log integrated exposures to dye concentrations; and converting the dye concentrations to a predicted reflectance spectrum.
Advantages that may be seen in implementations of the invention include one or more of the following: computation based on dye concentration is simple and direct; the predicted spectrum is an accurate model of photographic media and the development process; the model enables realistic proofing, preview, and global or selective manipulation of photographic images without physically exposing and developing a photographic print from a negative; experimentation with new algorithms for automatic color correction and investigation of material properties or printer characteristics are facilitated; and the determination of complex parameters is improved by using targets which are easy to produce and measure. Furthermore, given accurate parameters in the model, physical constraints of the photographic printing process may be relaxed and images better than those which are produced via analog printing may be obtained. For example, adjusting exposure range compression can avoid blocked-up shadows and burned-out highlights, and exposures that ignore secondary effects can result in more vibrant images. In addition, the simulated processing can be implemented as a computer program to provide a virtual photographic printer apparatus, complete with software controls mimicking those of real apparatus.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.